Many radar and communication systems need antennas with dual-polarization and dual-frequency capabilities for higher capacity data transfer. Microstrip patch antennas are often desirable antenna elements in such applications due to their low cost, low profile, light weight, and ease of fabrication characteristics. In recent years, there has been much research done in the field of designing dual-frequency and dual-polarization microstrip antenna arrays.
When designing dual-frequency, dual-polarized microstrip antenna arrays, many parameters of interest and the associated complexity both in design and fabrication are confronted. For instance, a complex feeding structure typically is required for reducing interconnect loss, feedline radiation, and cross-coupling. Substrate thickness can affect cross-polarization levels as well as bandwidth and efficiency. The distance of the antenna elements in the array can affect−3-dB beam width, directivity, and side-lobe levels besides impacting the overall size. Careful consideration needs to be given to avoid cross-coupling between the antenna arrays operating at different frequencies, blockage effects, and edge diffraction. Based on these and/or other considerations, it is challenging to achieve the aforementioned performance with a single layer structure.
In this regard, multilayer architectures have been considered. One such design of a dual-frequency, dual-polarized microstrip antenna array incorporating vertical integration was proposed by Granholm and Skou. This design incorporates C-band and L-band patches operating at 1.25 and 5.3 GHz, respectively. The C-band and L-band patches are located on metal layers separated by substrate layers of three distinct dielectric media, including foam.
Although there have been many reported examples of dual-frequency, dual-polarization microstrip antenna arrays on substrates, such as DUROID™, these designs are not always favorable for a radio frequency (RF) system-on-a-package (SOP) low-cost technology due to various undesirable substrate properties. Materials, like DUROID™, are often used in conjunction with low dielectric constant foam to realize multilayer configurations. Such composite multilayer structures are potentially subjected to greater stress due to coefficient of thermal expansion (CTE) mismatches, which can alter the dimensions of the structure.
Although low temperature co-fired ceramic (LTCC) technology is suitable for multilayer realizations of microwave circuits such as filters and other passives, LTCC technology is not ideal for antenna implementations. This is because antennas using high index materials such as LTCC typically result in pronounced surface wave excitation that can limit the impedance bandwidth, reduce the efficiency, and degrade the radiation pattern. One solution is to use micro-machined or suspended patch antennas albeit with increased fabrication cost and complexity. Another alternative is to use a hybrid integration scheme wherein different dielectric media can be integrated to control the effective index. However, such multilayer structures formed by integrating different materials tend to be subjected to greater stresses due to coefficient of temperature expansion (CTE) mismatches.